1. Field of the Invention
This invention relates generally to methods and compositions for cementing, and, more specifically to methods and compositions for cementing in deepwater and cold environments, as well as in environments where fluid influx or intrusion may be a problem during and after cementing. In particular, this invention relates to methods and compositions for well cementing utilizing mixtures of hydraulic cement and aluminum silicate.
2. Description of Related Art
Cementing is a common technique employed during many phases of wellbore operations. For example, cement may be employed to cement or secure various casing strings and/or liners in a well. In other cases, cementing may be used in remedial operations to repair casing and/or to achieve formation isolation. In still other cases, cementing may be employed during well abandonment. Cement operations performed in wellbores having relatively cool temperatures, ie., bottomhole circulating temperatures typically less than about 60.degree. F., may present particular problems, among other things, in obtaining good wellbore isolation. These problems are exacerbated in those cases where wellbore and/or formation conditions promote fluid intrusion during or after cement curing, including intrusion of water, gas, or other fluids.
Deepwater well operations typically include operations performed on offshore wells drilled in water depths over about 1,000 feet (especially in Northern latitudes), more typically greater than about 2,000 feet deep. Under deepwater conditions, relatively cool temperatures promoted by seawater, in some cases coupled with poorly consolidated formations often make the prevention of fluid intrusion during cementing a challenge. In such cases, relatively cool temperatures (typically less than about 60.degree. F., and more typically less than about 50.degree. F.) may slow cement curing or hydration, extending the transition time of a cement slurry. Transition time may be defined as the time required for a cement slurry to develop gel strength, or quantitatively as the time for a cement slurry gel strength to go from 100 lb/100 ft.sup.2 to 500 lb/100 ft.sup.2.
Because longer transition times means that the gel strength of a cement increases relatively slowly, there is more opportunity for intrusion of water or other fluids, such as oil or gas, to migrate through or displace a cement slurry. When such fluid migration occurs, channels, pockets or other cavities may form in the setting cement. Such cavities or channels may create a permanent flow passage or otherwise compromise the integrity of a cement sheath, such as exists between a pipe string and a formation. Furthermore, intrusion of a fluid such as water may dilute a cement slurry and thus prevent it from developing sufficient compressive strength. Fluid migration into a cement is typically more extensive when cement transition times are lengthened because although the cement column in a wellbore has typically built enough gel strength to support itself and to thereby reduce hydrostatic pressure on the surrounding formation, it has not developed sufficient gel strength to prevent fluid hydration. Although reduced gel strength, extended transition times, and fluid intrusion during cement curing is commonly encountered in deepwater completions, such problems may also be encountered in any wellbore having relatively cool formation temperatures, such as in wellbores drilled in cool or cold climates.
In those cases where formation sands are overpressured by fluids such as gas and/or water, fluid intrusion into the setting cement during the cement transition time may be a particular problem. In this regard, shallow formations in deepwater wells typically are unconsolidated, making them weak, prone to fracture, and prone to producing relatively high flows of water. Such a problem may be further exacerbated in those situations in which a relatively lightweight cement slurry is required. Such situations include those in which formations are susceptible to fracture, such as naturally weak or unconsolidated formations, or those with reduced bottom-hole pressures. Lightweight cements typically have longer transition times at relatively cool formation temperatures. Such cements are often referred to as "water extended cement slurries." Due to the relatively long transition times of water extended or lightweight cement slurries, there is increased opportunity for fluid intrusion and cement contamination. Such contamination may result in the loss of formation isolation and/or in casing damage. Resulting cement job failures may result in many undesirable consequences, such as the need for expensive remedial work, increased rig time, loss of production, and/or loss of the wellbore itself.
In cold weather regions, such as the Arctic, the temperature of shallow formations may not exceed 32.degree. F. for several hundred feet of depth. Such formations are typically referred to as "permafrost" which denotes a permanently frozen subsurface formation. Depending on the location, a permafrost or frozen section may extend from a few feet to depths greater than about 1500 feet. In such situations, even where fluid intrusion is not a problem, a cement slurry may not have the opportunity to set and provide needed strength before it freezes.
In some wellbores, gas intrusion may be a particular problem during and after cementing. Such wellbores include, for example, those where a wellbore penetrates a gas formation having a pressure corresponding to a first pressure gradient and a relatively underbalanced permeable zone having a pressure corresponding to a second pressure gradient that is lower than the first pressure gradient. In such cases, hydrostatic pressure exerted by the cement slurry may keep gas intrusion from occurring while the cement is still fluid. However, due to chemical hydration of the slurry and/or dehydration of the slurry across the permeable zone, the pore pressure of the slurry may decrease below the gas pressure in the reservoir allowing the gas to enter the cement. This underbalanced pressure may result, for example, in gas channeling to the surface or to another lower pressure permeable zone.